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Different Types of Voltmeters

Issue:

This document applies to all APC product families that produce a stepped approximated sine wave (a.k.a. quasi wave) when operating on-battery. When these units are on-line, they pass through the sine wave coming from the utility power.

Product Line:

Back-UPS and Back-UPS PRO

Environment:

N/A

Cause:

This discussion is a result of the fact that a Non-True RMS meter will read anywhere from 80 - 90 Vac from the output of a 120 Vac APC UPS (or 170 - 180 Vac from the output of a 230 Vac APC UPS) producing a stepped approximated sine wave while operating on-battery. This is not an issue when the UPS is operating on-line.

Resolution:

There are generally two types of meters: ""Average responding"" and ""True RMS"" meters. Average responding meters are more commonly used. True RMS meters tend to be more expensive. If the meter does not have ""TRUE RMS"" written right on the front, it is most likely NOT a True RMS meter.

The issue that arises with a non-True RMS or average responding meter is whether it is measuring the output from a linear load or a nonlinear load. Linear loads include but are not limited to devices such as light bulbs, incandescent lamps and resistive heaters. Nonlinear loads include devices like computers. When measuring the output of nonlinear loads, the average responding meter will typically read LOW. True-RMS meters are most effective when measuring environments with harmonics. When a waveform is distorted from a standard sine wave (""the fundamental""), an average responding meter may produce readings that are misleadingly incorrect. A stepped approximated sine wave appears distorted when compared to a true sine wave; therefore, a reading will produce incorrect results.

When an average responding meter is measuring the stepped approximated output from an APC UPS while operating on battery, this meter will also read low. The wave shape generated is similar to what the meter would see from a nonlinear load, hence the averaging calculation that the meter makes will be miscalculated." This document applies to all APC product families that produce a stepped approximated sine wave (a.k.a. quasi wave) when operating on-battery. When these units are on-line, they pass through the sine wave coming from the utility power.